1. Field of the Invention PA1 2. Prior Art
The present invention relates to a musical tone synthesizing apparatus which is capable of synthesizing sounds of non-electronic musical instruments by using a closed-loop circuit.
The musical tone synthesizing apparatuses, which are conventionally known and are disclosed by U.S. Pat. No. Re. 31,004 and Japanese Patent Laid-Open Publication No. 3-163597, are designed to synthesize the sounds of the non-electronic musical instruments by simulating their tone-generation mechanisms. Among them, the musical tone synthesizing apparatus which is designed to simulate sounds of stringed instruments utilizes a closed-loop circuit containing a filter and a delay circuit. Herein, the filter (e.g., low-pass filter) is provided to simulate a reverberation loss of the string, while the delay circuit is provided to simulate a propagation delay which is occurred when vibrating the string. Now, an excitation signal corresponding to an impulse or the like is introduced into the closed-loop circuit in response to the vibration applied to the string; and then, the excitation signal circulates through the closed-loop circuit. In this case, the excitation signal circulates through the closed-loop circuit once in a duration corresponding to a vibration period of the string. While circulating through the closed-loop circuit, a frequency band of the excitation signal is limited by the low-pass filter. Thereafter, the signal circulating through the closed-loop circuit is picked up as a musical tone signal which simulates the sound of the stringed instrument.
In the above-mentioned musical tone synthesizing apparatus which simulates the tone-generation mechanism of the stringed instrument, a delay time of the delay circuit and characteristics of the low-pass filter are respectively adjusted so as to synthesize the sounds of the non-electronic stringed instruments. The non-electronic stringed instrument represents a string-plucking-type instrument such as a guitar, a string-striking-type instrument such as a piano and the like.
Meanwhile, the guitar produces a decay sound which is naturally attenuated in a lapse of time. When simulating the decay sound by the aforementioned musical tone synthesizing apparatus, a loop gain of the closed-loop circuit is set at a specific value such as "0.9995", for example. This value enables the musical tone synthesizing apparatus to produce a natural decay sound whose sounding time is approximately ranged from three seconds to ten seconds. Actually, however, such sounding time may be varied by a tone pitch (i.e., delay time) or some parameters of the low-pass filter which are set due to the reverberation loss to be occurred.
When reducing the above-mentioned value to "0.95", the sounding time of the musical tone (i.e., decay sound) to be produced is extremely reduced to 0.1 second, for example. Such extreme reduction of the sounding time is occurred based on the fact in which when a frequency of the musical tone is set at 1 KHz, the musical tone is subjected to gain adjustment using the loop gain of 0.95 by one-thousand times in one second. This fact proves that a numeric value of the loop gain greatly affects the sounding time of the musical tone to be produced. In order to achieve a fine adjustment in an attenuation time (corresponding to the sounding time of the decay sound), it is necessary to provide a specific resolution by which four to six digits below the decimal point in the decimal notation can be clearly set. When realizing the musical tone synthesizing apparatus having the above-mentioned resolution by the digital system, 16-bit data processing system should be required, because the 16-bit data can achieve a large resolution which represents 65536 stages.
In the meantime, when the loop gain ranges from "0" to "0.9", the musical tone to be produced cannot be sustained even for a very short period of time. Hence, those values ranging from "0" to "0.9" are hardly used for the loop gain which is a fundamental parameter for the musical tone synthesis. In short, the aforementioned high resolution is not required for the loop-gain parameter whose value is set in a range of those values. Further, it requires much working time to adjust the attenuation of the musical tone by use of the loop-gain parameter having a high resolution which is set by a five-digit number such as "65536". For this reason, it is demanded to realize the above-mentioned loop-gain parameter by use of 7-bit data having a resolution of "128" stages, because popular synthesizer systems are configured to use the 7-bit data. In order to do so, a conversion table is provided especially for the loop-gain parameter, which is different from the other parameters. According to the function of the conversion table, the resolution can be raised as long as the loop-gain parameter is set in a specific value range, whereas the resolution is reduced when the loop-gain parameter is in a range of the values which are not substantially used for the musical tone synthesis. Therefore, most of the "128" stages represented by the 7-bit data are used for the loop-gain parameter which is in a range between "0.99" and "1", while the remaining stages are used for the loop-gain parameter which is in a range between "0.99" and "0". For example, "100" stages are used for the former loop-gain parameter, while tens of the stages are used for the latter loop-gain parameter.
By the way, an actual variation manner of the loop-gain which is used when actually playing the electronic musical instrument is very complicated. In the case of the so-called feedback performance of the electric guitar, the loop gain is normally set at or set above "1". In order to synthesize such electric-guitar sounds, the loop-gain parameter is set above "1" (e.g., "1.2"). In such case, an oscillation of the closed-loop circuit may be grown so that the tone volume becomes larger and larger. However, due to the effects of the non-linear characteristics of the electronic musical instrument and distortion circuits, the waveforms are deformed (or clipped), so that the tone volume is eventually converged upon an appropriate level.
As described heretofore, the actual electronic musical instruments use several kinds of values as the loop-gain parameter. In some cases, the loop-gain parameter is set around "1", or the loop-gain parameter is varied in a complicated manner. In the conventional apparatus, the loop gain is controlled by use of a single parameter. In such case, however, a complicated setting operation should be required for the loop gain.